In an electrical communications system, it is sometimes advantageous to transmit information signals (e.g., video, audio, data) over a pair of conductors (hereinafter a “conductor pair” or a “differential pair” or simply a “pair”) rather than a single conductor. The signals transmitted on each conductor of the differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors. This transmission technique is generally referred to as “balanced” transmission. When signals are transmitted over a conductor such as a copper wire in a communications cable, electrical noise from external sources such as lightning, computer equipment, radio stations, etc. may be picked up by the conductor, degrading the quality of the signal carried by the conductor. With balanced transmission techniques, each conductor in a differential pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both conductors of the differential pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair; thus the noise signal is cancelled out by the subtraction process.
Many communications systems include a plurality of differential pairs. For example, high speed communications systems that are used to connect computers and/or other processing devices to local area networks and/or to external networks such as the Internet typically include four differential pairs. In such systems, the conductors of the multiple differential pairs are usually bundled together within a cable, and thus necessarily extend in the same direction for some distance. Unfortunately, when multiple differential pairs are bunched closely together, another type of noise referred to as “crosstalk” may arise.
“Crosstalk” refers to signal energy from a conductor of one differential pair that is picked up by a conductor of another differential pair in the communications system. Typically, a variety of techniques are used to reduce crosstalk in communications systems such as, for example, tightly twisting the paired conductors (which are typically insulated copper wires) in a cable, whereby different pairs are twisted at different rates that are not harmonically related, so that each conductor in the cable picks up approximately equal amounts of signal energy from the two conductors of each of the other differential pairs included in the cable. If this condition can be maintained, then the crosstalk noise may be significantly reduced, as the conductors of each differential pair carry equal magnitude, but opposite phase signals such that the crosstalk added by the two conductors of a differential pair onto the other conductors in the cable tends to cancel out. While such twisting of the conductors and/or various other known techniques may substantially reduce crosstalk in cables, most communications systems include both cables and communications connectors that interconnect the cables and/or connect the cables to computer hardware. Unfortunately, the communications connector configurations that were adopted years ago generally did not maintain the conductors of each differential pair a uniform distance from the conductors of the other differential pairs in the connector hardware. Moreover, in order to maintain backward compatibility with connector hardware that is already in place, the connector configurations have, for the most part, not been changed. As a result, many current connector designs generally introduce some amount of crosstalk.
In particular, in many conventional connectors, for backward compatibility purposes, the conductive elements of a first differential pair in the connector are not equidistant from the conductive elements that carry the signals of a second differential pair. Consequently, when the conductive elements of the first pair are excited differentially (i.e., when a differential information signal is transmitted over the first differential pair ), a first amount of signal energy is coupled (capacitively and/or inductively) from a first conductive element of the first differential pair onto a first conductive element of the second differential pair and a second, lesser, amount of signal energy is coupled (capacitively and inductively) from a second conductive element of the first differential pair onto the first conductive element of the second differential pair. As such, the signals induced from the first and second conductive elements of the first differential pair onto the first conductive element of the second differential pair do not completely cancel each other out, and what is known as a differential-to-differential crosstalk signal is induced on the second differential pair. This differential-to-differential crosstalk includes both near-end crosstalk (NEXT), which is the crosstalk measured at an input location corresponding to a source at the same location, and far-end crosstalk (FEXT), which is the crosstalk measured at the output location corresponding to a source at the input location. NEXT and FEXT each comprise an undesirable signal that interferes with the information signal. In many connector systems, a plurality of differential pairs will be provided, and differential-to-differential crosstalk may be induced between various of these differential pairs.
A second type of crosstalk, referred to as differential-to-common mode crosstalk, may also be generated as a result of, among other things, the conventional connector configurations. Differential-to-common mode crosstalk arises where the first and second conductors of a differential pair, when excited differentially, couple unequal amounts of energy on both conductors of another differential pair where the two conductors of the victim differential pair are treated as the equivalent of a single conductor. This crosstalk is an undesirable signal that may, for example, negatively effect the overall channel performance of the communications system.
Cross-connect wiring systems such as, for example, 110-style and other similar cross-connect wiring systems are well known and are often seen in wiring closets terminating a large number of incoming and outgoing wiring systems. Cross-connect wiring systems commonly include index strips mounted on terminal block panels which seat individual wires from cables. A plurality of 110-style punch-down wire connecting blocks are mounted on each index strip, and each connecting block may be subsequently interconnected with either interconnect wires or patch cord connectors encompassing one or more pairs. A 110-style wire connecting block has a dielectric housing containing a plurality of double-ended slotted beam insulation displacement contacts (IDCs) that typically connect at one end with a plurality of wires seated on the index strip and with interconnect wires or flat beam contact portions of a patch cord connector at the opposite end.
Two types of 110-style connecting blocks are most common. The first type is a connecting block in which the IDCs are generally aligned with one another in a single row (see, e.g., U.S. Pat. No. 5,733,140 to Baker, III et al., the disclosure of which is hereby incorporated herein in its entirety). The second type is a connecting block in which the IDCs are arranged in two rows and are staggered relative to each other (see, e.g., GP6 Plus Connecting Block, available from Panduit Corp., Tinley Park, Ill.). In either case, the IDCs are arranged in pairs within the connecting block, with the pairs sequenced from left to right, with each pair consisting of a positive polarized IDC designated as the “TIP” and a negatively polarized IDC designated as the “RING.”
The staggered arrangement results in lower differential-to-differential crosstalk levels in situations in which interconnect wires (rather than patch cord connectors) are used. In such situations, the aligned type 110-style connecting block relies on physical separation of its IDCs or compensation in an interconnecting patch cord connector to minimize unwanted crosstalk, while the staggered arrangement, which can have IDCs that are closer together, combats differential-to-differential crosstalk by locating each IDC in one pair approximately equidistant from the two IDCs in the adjacent pair nearest to it; thus, the crosstalk experienced by the two IDCs in the adjacent pair is essentially the same, with the result that its differential-to-differential crosstalk is largely canceled.
These techniques for combating crosstalk have been largely successful in deploying 110-style connecting blocks in channels supporting signal transmission frequencies under 250 MHz. However, increased signal transmission frequencies and stricter crosstalk requirements have identified an additional problem: namely, differential-to-common mode crosstalk. This problem is discussed at some length in co-pending and co-assigned U.S. patent application Ser. No. 11/044,088, filed Mar. 25, 2005, the disclosure of which is hereby incorporated herein in its entirety. In addition, differential-to-differential crosstalk levels generally increase with increasing frequency, and conventional 110-style cross connect systems may not provide adequate differential-to-differential crosstalk cancellation at frequencies above 250 MHz.